Screwstrips are known whereby the screws are connected to each other by a retaining belt of plastic material. Screws in such strips are engaged by a bit of a screwdriver and then screwed into the workpiece. In the course of the bit engaging the screw and/or driving the same into the workpiece, the screw becomes detached from the plastic strip.
Known screwstrips of this type are disclosed in Canadian Patent 1,040,600 issued Oct. 17, 1978 to Schwartz, U.S. Pat. No. 4,167,229 to Reich et al, issued Sep. 11, 1979, U.S. Pat. No. 4,930,630 to Habermehl, issued Jun. 5, 1990, U.S. Pat. No. 5,758,768 to Habermehl, issued Jun. 2, 1998 and U.S. Pat. No. 5,819,609 to Habermehl, issued Oct. 13, 1998, the disclosures of which are incorporated herein by reference.
Screws carried by such strips are adapted to be successively incrementally advanced to a position in alignment with and to be engaged by a bit of a reciprocating, rotating power screwdriver and screwed into a workpiece. In the course of the bit engaging the screw and driving it into a workpiece, the screw becomes detached from the plastic strip leaving the strip as a continuous length.
In the use of such collated strips in screwdrivers, the strip serves a function of assisting in guiding the screw into a workpiece and, to accomplish this, the strip is retained against movement towards the workpiece. In the strip, each screw to be driven has its threaded shaft threadably engaged in a threaded sleeve of the strip such that on the screwdriver engaging and rotating each successive screw, the screw turns within the sleeve which acts to guide the screw as it moves forwardly into threaded engagement into the workpiece. Preferably, only after the tip of the screw becomes engaged in the workpiece, does the head of the screw come into contact with the sleeves. Further forward movement of the screw into the workpiece then draws the head downwardly to engage the sleeve and to rupture the sleeve by reason of the forward movement of the head with the strip retained against movement towards the workpiece. The sleeve preferably is configured to have fragible straps which break on the head passing through the sleeve such that strip remains intact as a continuous length. Since the strip is a continuous length, on advancing the strip with each successive screw to be driven, it necessarily results that portions of the strip from which each screw has been driven are also advanced to exit from the power screwdriver.
Known power screwdrivers for driving such collated strips include U.S. Pat. No. 4,146,871 to Mueller, issued Mar. 27, 1976, U.S. Pat. No. 5,568,753 to Habermehl, issued Oct. 29, 1996 and U.S. Pat. No. 5,870,933 to Habermehl, issued Feb. 16, 1999, the disclosures of which are incorporated herein by reference. Such known power screwdrivers include a rotatable and reciprocally moving screwdriver shaft which is turned in rotation by an electric motor. A screwdriving bit forms a forwardmost portion of the shaft for engaging the head of each successive screw as each screw is moved into a driving position, axially aligned under the screwdriver shaft.
Screws are known in which the shaft of the screw is threaded over substantially its entire length with identical threads. The present inventor appreciated that such screws have a number of disadvantages. Firstly, in driving the screws fully into a workpiece such as wood, resistance to driving increases as an increased length of the screw becomes engaged in the workpiece. For example, in hardwood or in almost any wood when long screws are used, the resistance to driving can become substantial and may result in torques being required to advance a screw which exceed the strength in the screw and result in the screw snapping. Secondly, when a screw is used to secure one workpiece to another, once the threads of the screw are engaged in both workpieces, it is not possible to draw the workpieces closer together by tightening the screw unless the thread grooves formed in the workpiece through which the screw passes entirely can be stripped.
Partially threaded screws are known which overcome these disadvantages by having a shank which is not threaded in an intermediate portion of the shank forward of the head and rear of a forward threaded portion. The intermediate portion of the shank typically is cylindrical without any threads thereon or, in some cases, has been known to be provided with a plurality of annular rings, the annular rings which extend radially outwardly an extent less than that of the threads of the forward threaded portion. These plurality of concentric spaced annular rings overlie the smooth shank and provide some increased engagement between the workpiece and the screw rearward of the threads. These rings have the disadvantage of being formed in a separate manufacturing step from the threads. The present inventor has appreciated that difficulties arise with such partially threaded screws including, amongst other things, difficulties with use of these partially threaded screws in collated screwstrips.
One disadvantage arises in that the preferred location for the holding strip to engage the shank is intermediate the head and the tip. However, this is the same location where there is an absence of threads on the partially threaded screws. Having the holding strip engage the screw where there is no threads has been appreciated by the applicant to prevent, on an initial rotation of a screw on engagement with a bit, from causing the screw to advance through the holding strap by threaded engagement between the screw and the holding strap. As well, where the holding strap engages a partially threaded screw which has a smooth cylindrical shank, difficulties arise in the plastic holding strap frictionally engaging the smooth cylindrical shank with sufficient friction to prevent the screws from becoming axially displaced in manufacture and handling of the screwstrip. With partially threaded screws which have the concentric spaced annular rings over the intermediate shank portion, where the holding strap is about such rings, then significant difficulties are experienced in attempting to drive the screw forwardly through the holding strip as the engagement of the concentric rings in the holding strip positively resists such advance.